Related I think some of the risk factor is vector related along with bodily reactions. Drinking too much leads to vomitting - while in the wrong circumstance potentially lethal it is expelled. One can't sneeze out cocaine or shoot heroin out of your veins autonomously if the body had too much.
Dose standardization does clearly help judging by the nasty side effects of overdosd deaths spiking after pill mills were shut down. It is not a panacea however - tolerance varies as well. It is why trying to get clean can be ironically dangerous. Things start going south and they try their old "standard dose" again after losing tolerance when it is enough to kill a 300lb man with no tolerance built up.
I wonder if legalizing weaned rehab would help. Like say cutting their dose by 25% repeatedly.
On a related note I wonder what role genetics play with it. It would be nice if susceptibility could be read from a genetic test so they could say "Don't worry about restricting opiates for him - he'll get weakened and would always want to get off them asap anyway but he shouldn't drink alcohol when stressed or take cough syrup but should avoid stimulants for their personal addictive potential."
I know personally from legitimate medical usage there are high potential for abuse drugs that for me have me wondering how anyone could consider it fun.